## Ada Lovelace Day — Henrietta Leavitt

Today is Ada Lovelace Day, “an international day of blogging to draw attention to women excelling in technology.” I — along with more than a thousand other people — have pledged to write about a female role model in technology.

Ada Lovelace was Byron’s daughter and worked with computer pioneer Charles Babbage on his “Computing Engines” — and is widely thought of as the first computer programmer. A reconstruction of the “Difference Engine” is on view at the Science Museum around the corner from here, and if you’re reading this on 24 March, you can go and talk to Ada herself!

But I want to talk not about a programmer, but a computer. That is, a computer named Henrietta Swan Leavitt. In the early 20th Century, some (always male) astronomers had batteries of (almost always female) “computers” working for them, doing their calculations and other supposedly menial scientific work.

Leavitt — who had graduated from Radcliffe College — was employed by Harvard astronomer Charles Pickering to analyze photographic plates: she counted stars and measured their brightness. Pickering was particularly interested in “variable stars”, which changed their brightness over time. The most interesting variable stars changed in a regular pattern and Leavitt noticed that, for a certain class of these stars known as Cepheids, the brighter ones had longer periods. Eventually, in 1912, she made this more precise, and to this day the “Cepheid Period-Luminosity Relationship” remains one of the most important tools in the astronomers box.

It’s easy enough to measure the period of a Cepheid variable star: just keep taking data, make a graph, and see how long it takes to repeat itself. Then, from the Period-Luminosity relationship, we can determine its intrinsic luminosity. But we can also easily measure how bright it appears to us, and use this, along with the inverse-square relationship between intrinsic luminosity and apparent brightness, to get the distance to the star. That is, if we put the same star twice as far away, it’s four times dimmer; three times as far is nine times dimmer, etc.

This was just the technique that astronomy needed, and within a couple of decades it had led to a revolution in our understanding of the scale of the cosmos. First, it enabled astronomers to map out the Milky Way. But at this time, it wasn’t even clear whether the Milky Way was the only agglomeration of stars in the Universe, or one amongst many. Indeed, this was the subject of the so-called “great debate” in 1921 between American astronomers Harlow Shapley and Heber Curtis. Shapley argued that all of the nebuale (fuzzy patches) on the sky were just local collections of stars, or extended clouds of gas, while Curtis argued that some of them (in particular, Andromeda) were galaxies — “Island Universes” as they were called — like our own. By at least some accounts, Shapley won the debate at the time.

But very soon after, due to Leavitt’s work, Edwin Hubble determined that Curtis was correct: he saw the signature of Cepheid stars in (what turned out to be) the Andromeda galaxy and used them to measure the distance, which turned out to be much further away than the stars in the galaxy. A few years later, Hubble used Leavitt’s Period-Luminosity relationship to make an even more startling discovery: more distant galaxies were receding from us at a speed (measured using the galaxy’s redshift) proportional to their distance from us. This is the observational basis for the Big Bang theory of the Universe, tested and proven time and again in the eighty or so years since then.

Leavitt’s relationship remains crucial to astronomy and cosmology. The Hubble Space Telescope’s “Key Project” was to measure the brightness and period of Cepheid stars in galaxies as far away as possible, determining Hubble’s proportionality constant and set an overall scale for distances in the Universe.

The social situation of academic astronomy of her day strongly limited Leavitt’s options — women weren’t allowed to operate telescopes, and it was yet more difficult for her as she was deaf, as well. Although Leavitt was “only” employed as a computer, she was eventually nominated for a Nobel prize for her work — but she had already died. We can only hope that the continued use of her results and insight to this day is a small recompense and recognition of her life and work.